1. Field of the Invention
The invention relates generally to leak detection devices and more particularly to a leak detection apparatus employing a means for sensing the present of an inert probe gas passing from a sealed annular chamber to a vacuumized inner chamber.
2. Description of the Prior Art
In the field of vehicle manufacturing, most motor cars and trucks are fitted with drum wheels. The source of the drum wheels may be the vehicle manufacturer or a vendor supplier, each of whom are required to provide wheels that satisfy certain engineering specifications. One such specification is that wheels that are mounted on vehicles or supplied to wholesale-retail suppliers should be free of structural flaws that result in leakage of the pneumatic medium air. Certain minute structural flaws may result in very slow leakage, evidence of which may not appear until after the wheel is in use. In standard warranties covering material and workmanship, the manufacturer or vendor supplier would be obligated to repair or replace the defective wheels resulting in considerable expense for shipping, labor and material to the supplier and obvious inconvenience to the customer.
In U.S. Pat. No. 3,721,117 issued Mar. 20, 1973 to Ford et al., a method and apparatus is disclosed for detecting leaks in pneumatic wheels. Note that this reference has a number of embodiments, however, each generally provide that a tire 14 is mounted on a rim 12. The complete assembly 10 includes both the rim 12 and the tubeless tire 14. Also note that Ford et al. inflates the tire 14 with a probe gas which is preferably a mixture of air and helium. In one embodiment, the entire wheel assembly 10 is enclosed within a heat shrinkable plastic envelope 46. With the plastic envelope 46 forming a seal around a line 60/62, any volume of probe gas which leaks through the valve stem 38 or through the rim 12 will be contained by the envelope 46. Subsequently, the envelope 46 is punctured with a conventional helium detector 66 to test the space within the chambers for the presence of helium.
In the embodiment of FIG. 6, a vacuum pump 116 is connected through the pipe 114 to a one-way valve 108. After removal of the air, the covering placed over the aperture 106 is removed and the detector 118 is utilized to detect whether the helium has leaked from the wheel assembly 112 into the space bounded by the upper surface of the support platform 82 or flexible sheet 86. The use of the disposable plastic envelope 46 to provide a sea tends to decrease the reproducibility of the results together with the tire employed to contain the probe gas infers a non-automated process of wheel rejection or approval. The method and apparatus is concerned primarily with detecting leakage in the rim-tire interface and about the valve stem and not through the wheel structure itself. The use of a vacuum pump shown in FIG. 6 is for evacuating the volume within the plastic envelope 46 and not for establishing the high vacuum required when employing a mass spectrometer.
In U.S. Pat. No. 4,055,984 issued Nov. 1, 1977 to Marx, a device is disclosed for detecting leaks in flexible articles which is termed leak detection by vacuum. The procedure involves leak detection from the outside to the inside on articles with easily deformable walls. Leak detection is performed on an article 4 which is mounted within a dome 19. A vacuum is drawn into the space 66 through a nipple 18 provided in a closure 14 to the test gas detector. The article 4 is sprayed with a test gas and where a leak is present, the test gas penetrates into the evacuated space 6b. This procedure also infers a non-automated process which employs disposable hoods of plastic employed to contain a gas mixture or to trap an escaping probe gas.
In U.S. Pat. No. 3,174,329 issued Mar. 23, 1965, to Kaufmann et al., there is disclosed a method for testing ordinance (projectile) seals. The underwater ordinance 10 is first charged internally by a gaseous composition such as Freon-12 and then placed into an enclosed test chamber 11. A ventilation blower 14 is energized to circulate ambient air through the chamber 11 via a ventilation duct 13. A sampling probe 15 of a detector 16 is inserted into a fitting 18 in the duct 13 to obtain a sample of air for analysis purposes. The ordinance (projectile) is portrayed as a hermetically sealed unit which is comprised of a flexible plastic material which is collapsible for storage purposes. Note that this invention does not present the problems associated with leak testing an open container and a vacuum system is not involved.
In U.S. Pat. No. 3,813,923 issued June 4, 1974, to Pendleton, there is disclosed an adaptive fixture for leak testing of containers. The system 10 illustrates a diaphragm-type enclosure employed for leak testing a container 11 such as an automobile fuel tank and includes a top frame 12 having a hollow portion 13. A second frame 14 exists which has another hollow portion 15. A first flexible diaphragm 16 is mounted to the frame 12 and extends across the hollow portion 13. The diaphragm 16 is spaced from the inside surface 17 of the frame 12 and defines a first pressure chamber 18 between the surface 17 and the diaphragm 16. By opening the valve 43 in the line an allowing the pressurized gas to communicate within the chambers 18, 21, the chambers 18, 21 are pressurized and force the diaphragms 16, 19 to conformingly fit around and support the container 11. This action reduces the volume of the enveloping container 11 and also provides external pressure to the container 11 to counteract internal pressure supplied by a pumping of a probe gas into the fixture being tested. Note the diaphragms 16 and 19 act as a stabilizer support and the test container does not become an integral part of the adaptive fixture.
In U.S. Pat. No. 3,968,675 issued July 13, 1976 to Briggs, there is disclosed a method and system for preparing a mass spectrophotometer leak detector system tuned to detect the presence of a helium gas. A roughing pump 8 is operated with a valve 16 closed and a valve 18 open. A high vacuum pump 3 reduces the total pressure in the mass spectrophotometer 1 to operating levels. At this point, the mass spectrophotometer 1 is energized while the air inlet valve 14 is physically removed and the inlet fitting 12 is coupled to a leak test arrangement. If the structure being leak tested developes a leak, the helium test gas will pass through the leak and the presence of helium will be indicated on the mass spectrophotometer 1. The Briggs '675 patent appears to address the specific problem of providing a reference valve to ascertain the partial pressure of helium within an air volume. This system appears to quantize the amount of helium gas which has passed through the system and not a device that indicates the presence of any probe gas.
In U.S. Pat. No. 3,855,844 issued Dec. 24, 1974, to G. M. Craig, there is disclosed a leak detection system for sealed beam headlamp units, each of which has a gas fill including helium, wherein a unit is placed in a test chamber which is evacuated to draw the gas fill through any unit leakage paths into the chamber. A first pressure gauge coupled to the chamber performs a first leakage check. If the pressure is above a predetermined value indicative of a high rate of leakage, the unit is rejected, otherwise, a metered volume of carbon dioxide is injected into the chamber for admixing with any leakage gas fill. A second leakage check test is conducted rejecting those units with pressures above a predetermined high leakage rate but less than the rate of the first check. The unit is rejected, otherwise the uncondensed gas fill is valved to a mass spectrometer for a final leakage test. This leak detection system is applicable to the sealed enclosure and not to an open container device.
Finally, in U.S. Pat. No. 3,027,753 issued Apr. 3, 1962, to A. J. Harder, Jr., there is disclosed a leak detection device for detecting minute leaks in enclosed, sealed units. Specifically, the device detects leaks of any magnitude in a thermal insulation unit in which a charge of gas is contained within a hermetically sealed, flexible envelope of gas-impervious material. Also, the apparatus sequentially tests flexible walled sealed units of divergent sizes without a change of the setup of the machine required. Further, the resilient walled testing chamber closely conforms to the shape of the flexible walled unit under test to reinforce the unit walls and closely conform to the outer contours of the object under test to support the object against rupture by internal pressures but allow for minute amounts of leakage gas to reach the leak-detection apparatus. The '753 patent is not applicable to open container devices subject to leak detection.
The recent prior art also includes the following process for testing drum wheels for structural integrity and leakage of the pneumatic medium. A pneumatic test is conducted by clamping the wheel under test between two platens along the axial centerline of the wheel. The two platens each have elastomeric engaging discs on opposite interior faces or engaging surfaces. A chamber is therefore defined within the wheel cavity and between the two opposing platens. A compressive force of approximately 2,000 pounds per square inch (psi) is applied to the opposing platens to create a seal. The chamber is then pressurized to 55 psi plus or minus five pounds, wherein the wheel is submerged in a water bath and visually inspected for bubbles. The presence of bubbles indicates the passage of air from the interior defined chamber to the outer exposed wheel surface. The recent prior art devices also include various means for rotating the wheel to facilitate the search for air bubbles.
Such arrangements suffer from a plurality of deficiencies in addition to those attendant the prior art references discussed hereinabove. It has been the practice that the pneumatic pressure is applied to the normally exposed or outer surface of the drum wheel and any escaping bubbles are detected on the edge surface of the wheel normally covered by a tire. It should be appreciated that in the procedure just described, the pressurized air is forced in a direction opposite to those forces present under normal operating conditions. Furthermore, these prior art testing procedures and devices require that the wheel be placed in a fluid medium to facilitate detection of air leakage. Finally, the test procedure requires a subjective analysis or inspection by a human observer to decipher whether a bubble is forming on the surface of the wheel being tested. The inspector must discern a bubble originating from air leakage from a bubble caused by immersing the wheel into the fluid medium, a subjective test at best.
Additionally, a machine for leak testing wheels in the automotive industry employs a hydraulic press. The press captures the wheel under test between a top and a bottom platen providing an inner chamber within the structure of the wheel. A housing is lowered over the wheel to form an outer chamber that encloses the wheel. A vacuum is drawn on the outer chamber and a test gas such as helium is introduced into the inner chamber. If a leakage exists in the structure of the wheel, the helium-air mixture will pass from the inner chamber to the vacuumized outer chamber. A helium monitoring device is present in the outer chamber which announces the presence of the test gas if a leak exists.
A major problem with this testing scheme is that under normal operating conditions the gaseous medium would pass from the tire cavity (outer chamber) through an existing leak in the wheel into the area described as the inner chamber during test conditions. This movement of the gaseous medium in reality is just opposite to the movement of the gas mixture in the described leak test. Where leakage holes of microscopic size are involved, it is important to test the structure of the wheel in a manner consistent with normal operating conditions of the wheel. Particles of aluminum or dust resident in the wheel structure are capable of blocking a leakage path measured in micro dimensions when the gaseous mixture is passed in one direction. However, when the gaseous mixture is passed in the opposite direction, the microscopic particles may be dislodged and the leakage path is exposed. Thus, a leakage test that applies the gaseous mixture in a direction opposite to the normal flow of air through the wheel if a leak occurred is not reliable in testing for leaks.
The foregoing problems continue to exist and, as such, a significant need exists for an improved apparatus and method for leakage detection in drum wheels or otherwise tubular open ended assemblies wherein the wheel can be quickly, reproducibly and reliably tested for structural integrity and leakage.